The manufacture of certain compositions require the combination of two or more component fluid materials, often under high pressures in a mixhead to suitably meter and/or blend the component fluid materials, which mixhead employs a plunger reciprocating within a mixing chamber. If the plunger breaks or otherwise fails, the constituent fluid materials may not mix properly causing significant delay and additional expense in manufacturing operations. For example, during the manufacture of polyurethane foam accomplished by the mixing of a polyol with an isocyanate in such a mixhead, failure to cease operation promptly upon malfunction of the mixhead plunger results in blockages in the mixhead, and supply and return lines, necessitating difficult and potentially costly clean-up, new setup and down time. I have appreciated that monitoring select operational parameters to determine whether the mixhead plunger has failed and, if so, generating a suitable alarm, allows implementation of measures to minimize or eliminate such deleterious consequences.
In the past the flow of liquid or gas in a variety of dispensing and mixing systems has been monitored most commonly by measuring pressure at or following the point of dispensing or mixing, respectively, comparing such pressure with a preselected pressure magnitude or small range of pressure magnitudes, and triggering an alarm when such magnitude or small range of magnitudes were exceeded. For example, U.S. Pat. Nos. 4,430,886 and 4,668,948 present methods, apparatus and circuits for the sensing of a clogged paint spray nozzle by measuring paint pressure and comparing the magnitude of such pressure to preselected limit(s). Other applications in which fixed pressure limits have been used to monitor dispensing flow conditions include the pump alarm system of U.S. Pat. No. 3,050,003, the pneumatic warning systems of U.S. Pat. No. 3,223,068, the liquid lubricant pumping system of U.S. Pat. No. 3,730,297, the plasticizer portion of an injection molding machine in U.S. Pat. No. 3,750,134, and the fluid dosing circuit of U.S. Pat. No. 4,800,367.
Several exemplary applications involving mixing include the machine for making carbonated beverages depicted in U.S. Pat. No. 2,724,581, the apparatus for mixing two gases shown in U.S. Pat. No. 3,901,178, the apparatus for combining an additive to a monomer for the manufacture of plates shown in U.S. Pat. No. 3,999,686, and the lacquer mixer of U.S. Pat. No. 4,440,314.
The reason such applications as these may successfully monitor and compare pressure to a preselected fixed pressure magnitude or small range of pressure magnitudes is because pressure remains substantially constant throughout operation. Indeed, when there is a minor perturbation in pressure, the device and method has been designed to eliminate or mask such variations. For example, in U.S. Pat. No. 4,613,059 a spray nozzle for the application of an adhesive is shown to include a so-called pulse masking circuit 28 for suppressing all pressure variations before reaching a steady-state from which the comparative pressure magnitude may be taken.
However, where fluid materials are blended in a mixhead that employs a plunger reciprocating within a mixing chamber to suitably meter and/or blend the component fluid materials, operating parameters such as pressures may vary significantly as the plunger opens and closes each material inlet port. Thus, any effort at detecting a plunger malfunction by comparing such monitored pressures to preselected fixed limits would be unsuccessful, identifying false malfunctions and missing entirely some situations in which actual damage has occurred.